The invention relates to a pneumatically operated sleeve-type compressor bleed valve for a gas turbine engine supported with guide bearings to operate in a helical path.
Gas turbine engines often include a low-pressure compressor stage and a high-pressure compressor stage for pressurizing ambient air as it flows through the compressor flow path to the combustor and turbines. Under certain operating conditions it is necessary to moderate the air pressure at the discharge end of the compressor to address aerodynamic instabilities such as compressor stall or surge. In order to moderate the pressure it is conventional to open a compressor bleed valve that directs a portion of the pressurized air from the compressor flow path into a lower pressure region such as the bypass gas path or external ambient air.
U.S. Pat. No. 6,122,905 to Liu describes a conventional compressor bleed valve that is mounted rigidly through the engine to discharge flow from the compressor into a narrow area within the bypass flow duct. This pneumatic side valve arrangement is suitable for turboprop or turbo shaft engines since compressed air is exhausted directly to the ambient air flow. However in fan engines with bypass ducts, the valve protrudes through the bypass duct and concentrates the compressor discharge air flow in a narrow area within the bypass duct flow. The injection of additional compressed air into the bypass duct within a limited flow area creates airflow variations, and the location in the bypass duct partially obstructs the bypass flow.
An alternative bleed valve is shown in U.S. Pat. No. 6,161,839 to Walton et al. described as a xe2x80x9csleeve valvexe2x80x9d having an annular skirt or sleeve with sealing surfaces to impede air flow over a circumferential valve seat from discharge slots in the compressor flow path. Conventional sleeve-type valve assemblies include a bellcrank mechanism to axially translate the sleeve between open and closed positions. The conventional sleeve valve arrangement is very reliable, distributes air flow more uniformly and is compact. Such valves require an external hydraulic actuator and are mechanically complex due to a plurality of bellcrank actuators that are necessary about the periphery of the sleeve to provide uniform valve operation.
Sleeve-type valves require relatively short stroke movement of the valve plug in order to release substantial volumes of air from the compressor airpath. In contrast, individual pneumatically operated compressor bleed valves are often provided in multiples since they are capable of exhausting a relatively small air flow volume. A major disadvantage of the pneumatic compressor bleed valves conventionally used is that the pistons of the valves are usually guided by a single central pin or shaft or by the side walls of the piston itself. The radially operating valve pistons are subjected to substantial side loads from axially directed air flows which induces friction and over time reduces the response rate of the valve due to frictional wear.
It is an object of the present invention to provide a mechanically simple lightweight pneumatically operated sleeve type bleed valve that eliminates reliance on multiple individual activated valves or bulky external mechanical actuators.
It is a further object of the invention to provide a sleeve-type valve that is subjected to axial loads parallel to its operating direction and thereby eliminates detrimental side loading problems of the prior art.
Further objects of the invention will be apparent from review of the disclosure, drawings and description of the invention below.
The invention provides a pneumatically operated bleed valve in a containment housing, such as an axial or centrifugal compressor housing and exhausting to a bypass duct of a gas turbine engine, for example. The valve has an annular piston slidably mounted within an annular chamber concentric the longitudinal engine axis. An annular valve plug on the piston and an annular valve seat on the compressor housing defining a valve seal interface. A control air pressure conduit communicates between a portion of the annular chamber bounded by the piston and a source of control air pressure. Guide bearings mounted to the periphery of the piston and the housing have a helical guide surface concentric the engine axis
The result is a sleeve-type pneumatically actuated compressor bleed valve that is lightweight, compact and reliable due to simple axial actuation using an annular chamber with annular piston sealed therein. The piston is guided axially in a helical pattern with three supporting bearings and includes a skirt serving as a valve plug to engage a mating conical valve seat. The valve plug includes variable area orifices to permit a gradual increase and decrease to the volume of flow and flow resistance of the valve assembly.
An advantage of the annular valve plug and skirt is the ability to permit passage of a high volume of compressed air with very little axial motion due to the large peripheral or circumferential surface area that can be exposed, relative to individual pneumatically operated valves of the prior art. Guiding of the axial motion of the annular piston with three bearings in helical tracks provides stability to the piston and eliminates side loading since the major forces acting on the piston are axially oriented.